Each year people are injured or killed as a result of the chemical toxicity or explosive flammability of many hydrogen-containing gases such as natural gas and propane. Such hydrogen-containing gases are commonly used or found in a residence or workplace. A number of methods exist to detect hydrogen-containing gases but many of the methods suffer drawbacks when employed in residential and industrial applications.
U.S. Pat. No. 3,695,848 by Taguchi discloses an apparatus for detecting hydrogen-containing reducing gases, by heating an n- or p-type metal oxide semiconductor such as tin oxide to 150.degree. C. to 250.degree. C. Reducing gases such as H.sub.2, CO, C.sub.2 H.sub.2, CH.sub.4, C.sub.2 H.sub.5 OH, C.sub.3 H.sub.8 and C.sub.4 H.sub.10 lowers the resistance of the semiconductor through combustion heating. The decreased resistance permits more current to flow across the semiconductor which triggers an alarm at desired concentrations.
The device has a number of disadvantages. First, the device measures only those hydrogen-containing gases which are reducing. Toxic hydrogen-containing gases such as ammonia and sulfuric acid are not detected by the device. Second, the device requires a high power input to maintain the desired temperature of the ceramic insulator. As a result, it is not practical to power the device for long periods using a battery. Third, the device is incapable of detecting hydrogen-containing compounds in an oxygen depleted environment that will not support a combustion reaction or above the upper explosive limit of such compounds, which is a concentration of about 15% by volume for methane. Fourth, the device creates a risk of explosion for explosive hydrogen-containing gases such as natural gas and propane due to the high temperature of the semiconductor. Finally, the accuracy of the device may be decreased by the accumulation of residue on the semiconductor. In particular, grease generated in a kitchen environment may produce a carbon residue, short circuiting the sensor.
U.S. Pat. No. 4,194,115, by Whitehead, et al., discloses a device employing two charged particle detectors to determine the ratio of helium to hydrogen in a sample gas. The detectors measure the energy loss of the products of collisions between alpha particles on the one hand and either hydrogen or helium on the other. The detectors are aligned such that the high energy recoil protons from the collisions between alpha particles and hydrogen nuclei, but not helium particles, pass through the first detector and impact the second detector.
This device also suffers from a number of disadvantages. First, accuracy of the device's measurement is decreased by the device's use of baffles to limit the particle spectrum impacting the detectors to a specific predetermined forward scattering angle. Second, the device's use of coincidence counts between the two detectors to detect the ratio of helium to hydrogen is inherently inefficient as a result of the need for coincidence detection of two events. Third, since the barrier (e.g., the first detector) used to stop alpha or helium particles must be a semiconductor, the available thicknesses of the barrier are restricted. This may complicate the optimization of the barrier thickness to permit a maximum number of recoil protons and a minimum number of alpha or helium particles to pass through the barrier. In short, the barrier will either be too thin and permit additional alpha or helium particles to contact the second detector or too thick and fail to permit substantially all recoil protons to contact the second detector. Fourth, the device is designed for use in outer space and will not operate in air (oxygen and nitrogen). The heavier oxygen/nitrogen nuclei will scatter a large fraction of the primary alpha particles into the first detector creating inaccurate measurements. Finally, the device's use of separate detectors requires additional supporting circuitry which substantially increases the complexity and cost of the device.
There is also a need for a low cost detection device to detect radon and radon daughters in residential and industrial applications. In recent years, the detrimental effects of radioactive materials released into the atmosphere either from radioactive deposits underlying buildings or from materials used in the workplace have become an increasingly serious problem. Numerous people each year contract cancer as a result of such radioactive emissions.
U.S. Pat. No. 4,186,303, by Smith, et al., discloses an apparatus for detecting alpha particles from radon 222 and not radon 220 by enclosing the detector with a semi-permeable membrane which passes alpha particles from radon 222 but not from radon 220. The detector is designed to be placed in a hole in the earth to detect underground deposits of uranium and/or thorium. The apparatus has a number of disadvantages. First, it is designed only for use in detecting underground deposits of uranium and thorium and is not suitable for use in residential and industrial applications for the detection of radioactive materials released into the ambient atmosphere. Second, the apparatus detects only alpha particles from radon 222 and not other radioactive sources such as radon 220, which is a commonly encountered radioactive material in residential and industrial settings. Finally, the apparatus employs two dosimeters which, along with their supporting circuitry, substantially increase the complexity and cost of the device.
U.S. Pat. No. 4,811,714, by Simon et al., discloses an apparatus for continuous determination of gas-carried alpha activity caused by radioactive charge of thorium, uranium, platinum, and their decay products. The device employs a semiconductor sensor and claims improved response by the use of dummy circuitry and the electroplating of radon daughters onto the detector surface. The apparatus has a number of disadvantages. First, the dummy circuitry increases the complexity and cost of the device. Second, the counts arising from radon daughters plated onto the surface of the detector gives rise to inaccuracies in the radon count since the radon daughter concentration is not known a priori and can vary significantly. Finally, there are important limitations on the accuracy of the electrostatic precipitation method due to neutralization processes.
U.S. Pat. No. 4,960,998, by Peter, discloses an apparatus for determination of gas-carried alpha activity caused by the decay of thorium, uranium, plutonium and their decay products. The gas-carried alpha activity from the decay chains of radon 220 and radon 222 is subtracted from a total gas-carried alpha activity measured by the apparatus. The apparatus employs three alpha counters, one to measure total alpha activity, one to measure gas-carried alpha activity of Rn-220 and one to measure the gas-carried alpha activity of Rn-222. The apparatus also has a number of disadvantages. First, the apparatus is not intended to determine the full spectrum of alpha particles generated by radioactive sources, but subtracts out the alpha activity from the decay chains of radon 220 and radon 222. Second, the use of three separate detectors and their supporting circuitry substantially increases the complexity and cost of the device. Accordingly, the device is not suitable for residential and industrial applications which require an inexpensive apparatus to detect the full spectrum of alpha activity generated from radioactive sources.